The use of power amplifiers in transmitting radio frequencies (RF) signals has many applications, including but not limited to radiotelephone communications systems. In a radiotelephone communications system, there are multiple fixed site transceivers. Each fixed site transceiver is an interface between the line telephone system and multiple, portable, or mobile radiotelephone systems contained within a geographic area served by the fixed site transceiver. The fixed site transceiver and the radiotelephone communicate by sending and receiving FM modulated radio frequency signals to each other.
In an analog radiotelephone system, there is a large available RF spectrum available for radiotelephone communications. The available RF spectrum is channelized into relatively narrow segments of frequency. Upon request, each radiotelephone is allocated a frequency channel upon which to broadcast and receive information from the fixed site transceiver. This is known as a Frequency Division Multiple Access (FDMA) system. During a phone call, the radiotelephone transmitter would turn-on and remain on the fixed frequency for the entire time of the phone call. Thus, the transmitter and subsequently the power amplifier (PA) of the radiotelephone only needed to turn-on once and turn-off once for a given phone call which could last several minutes. The fixed site transceiver could also remain on for the duration of a phone call. In an FDMA system, since there are a limited number of turn-ons, the turn-on and turn-off requirements of the power amplifier are not very stringent. If a transmitter is turned on quickly, there is a momentary large burst of noise, however, since this would only occur once during a conversation, it would not substantively affect the communication system. Similarly, if the power amplifier is turned on slowly, there would be an imperceptible gap in the conversation at the beginning of a phone call. These problems are only inconveniences to the user and are not substantive system degrading problems.
In the newly proposed digital cellular radiotelephone systems, the power amplifier turn-on requirements have increased dramatically. The new systems use a time division multiple access (TDMA) communications system. In a TDMA communications system, 8 or 16 radiotelephones share a single 200 kilohertz wide channel on which to broadcast. Each radiotelephone in a channel is allocated one 577 microsecond (uS) time slot on a repetitive basis. During this time slot the radiotelephone ramps up the power amplifier to the proper frequency and power, sends the desired data, and ramps down the power amplifier so as not to disturb or interfere with the other users sharing the same frequency. Thus, the requirements for controlling the power amplifier are greatly increased.
The Group Special Mobile (GSM) recommendations ETSI/PT-12 05.05 (4.2.2 and 4.5.2), March, 1991 were developed to define a digital radiotelephone communications system. These recommendations were aware of the increased power amplifier requirements and have defined a time mask and a spectral frequency mask, as illustrated in FIGS. 7 and 8, which all radiotelephone equipment used in the system must meet. The specifications concerning the time and frequency masks demand the development of a very smooth ramp up of the PA and stringent time constraints.
Without these requirements, TDMA digital communications systems would not operate. If the PA is turned on too slow, even a few microseconds, severe damage to the data transmitted between the fixed site transceiver and the radiotelephone would occur. Turning a power amplifier on very quickly results in large spectrum burst causing interference with radiotelephones of the same or similar frequencies. Therefore, a power amplifier controller is needed which ramps the power amplifier up to the required power quickly and smoothly without causing a frequency noise burst or missing data.
Previously, a digital signal processor (DSP) has been used along with a digital-to-analog converter (D/A) to generate a ramp up of the PA as required by the GSM specifications. The Automatic Output Control (AOC) voltage generated by the DSP is fed into an integrator where it is compared to the output of an RF power detector. The difference between the AOC signal and the detector signal is fed into the control input of the power amplifier system. Under ideal conditions, this closed loop power amplifier system will adjust the control voltage until the RF control detector is equal to the AOC voltage. However, the system is far from ideal.
Two characteristics of the system lead to shortfalls in maintaining the time and spectral frequency masks. First, the detector has a finite range over which it detects RF from the output of the amplifier. Below this range, the detector outputs a small voltage, which does not correlate to the power changes of the amplifier. Under this condition, the control loop is open and the output power is in no way correlated to the AOC voltage. In fact, the integrator will force its output to the maximum negative voltage, because the detector voltage will be greater than the AOC voltage. Secondly, the exciter/power amplifier typically has a turn-on threshold which the integrator must meet before the ramp up waveform can be input to the amplifier.
Because of the shortfalls of the control loop, when the power amplifier becomes active, and the detector subsequently becomes active, the control loop closes and attempts to track the AOC voltage. At this point, the control loop will attempt to track the AOC voltage at its current level. This will result in a spectral frequency mask violation because of the sharp turn-on of the power amplifier 401 as shown in FIG. 4.
Therefore, a need exists for a power amplifier control loop which is adaptive to the requirements of each individual power amplifier and allows for a quick and smooth power ramp up.